Clinical multidrug resistance in the treatment of bacterial and fungal infections and cancer chemotherapy can result from expression of membrane-embedded efflux pumps that extrude cytotoxic molecules out of the cell. A subclass of these pumps consists of ATP binding cassette (ABC) transporters, ATP-powered traffickers of a wide range of molecules. The long term goal of this research is to define the protein motion that couples energy expenditure to solute translocation by ABC transporters. Roughly 5% of the Escherichia Co//genome encodes for ABC transporters;one of which, MsbA, transduces ATP energy to flip lipid A, the building block of the outer membrane, across the inner membrane. Its critical role in bacterial homeostasis, sequence and functional similarity to human multidrug transporters in conjunction with extensive crystallographic analysis make MsbA a biochemically and biophysically tractable model of clinically relevant ATP-coupled transport. We will use spectroscopic techniques to obtain direct structural and dynamic information on well-defined, key catalytic intermediates of MsbA in lipid bilayers and in the absence of conformational selectivity by crystal lattice forces. Thespecific aims willtest a mechanism of transport that envisions anATP- and substrate- regulated switch whereby transport occurs concomitantly with cycles of dimerization and dissociation of the nucleotide binding domains (NBDs). Spin labels will be systematically introduced into the protein sequence and their mobilities, accessibilites and pairwise proximities analyzed by electron paramagnetic spectroscopy (EPR) to 1) reconstruct the relative movements of the NBDs and 2) map conformational changes that reorient the substrate binding chamber. A novel aspect of this proposal is the use of complementary spectroscopic rulers with a 5-80A distance range to address controversial aspects of the crystal structures and add a dynamic dimension to these static snapshots. The proposed studies will bridge the current divide between structural and mechanistic models of ABC transporters and provide a unique dynamic perspective on a process of fundamental biochemical importance. In addition to controlling the pharmokinetic profile of xenotoxins, human ABC transporters play causative roles in a number of genetic disorders including cystic fibrosis. Understanding their mechanisms will aid in the design of new drugs to overcome resistance to chemotherapy and the development of therapeutic strategies for the inherited pathologies.